A low pressure EGR apparatus is known, in which an exhaust gas passage disposed downstream from a turbine of a turbocharger is connected to an intake gas passage disposed upstream from a compressor of the turbocharger so that a part of the exhaust gas is supplied as the EGR gas. In the case of the low pressure EGR apparatus, the EGR gas passes through an intercooler. Therefore, the moisture content (water), which is contained in the EGR gas, is condensed in some cases when the EGR gas is cooled by the intercooler. If the condensed water, which is produced as described above, flows into a cylinder and adheres to a spark plug, then the spark is hardly generated, and the combustion state is deteriorated in some cases. Further, if a large amount of the condensed water flows into the cylinder, the so-called water hammer phenomenon occurs, in which the reciprocating motion of the piston is impeded. Further, if the condensed water is produced, it is feared that the intercooler may be corroded. Further, if unburned fuel or particulate matter contained in the EGR gas adheres to the condensed water, it is feared that the cooling efficiency of the intercooler may be lowered.
In relation thereto, it is known that the supply of the EGR gas from a low pressure EGR apparatus is stopped, assuming that the condensed water is produced if the downstream end temperature of an intercooler is less than a predetermined temperature and the boost pressure is less than a predetermined pressure (see, for example, Patent Literature 1).
Further, it is known that the flow rate of the intake gas of an intercooler is controlled so as not to exceed the limit relative humidity which is calculated on the basis of the vapor pressure of the intake gas provided at the outlet of an intercooler and the flow rate of the intake gas passing through the intercooler (see, for example, Patent Literature 2).
Furthermore, it is known that an EGR valve is closed if the cooling water, which flows through an intercooler, has the temperature that is less than a temperature at which it is feared that the condensed water may be frozen (see, for example, Patent Literature 3).
Moreover, it is known that the condensed water removal control, in which a pressure difference (differential pressure) is generated between one end side and the other end side of a condensed water removal passage to thereby allow the condensed water to flow out from the condensed water removal passage to an intake gas passage disposed on the side downstream from an intake gas adjusting valve, is executed, but the condensed water removal control is prohibited if an internal combustion engine is in the idle state (see, for example, Patent Literature 4).
Moreover, it is known that a discharge unit, which makes it possible to discharge the condensed water, is provided at a bottom portion of a complex type heat exchanger (see, for example, Patent Literature 5).
However, when the supply of the EGR gas is stopped, then the production of the condensed water can be suppressed, but it is impossible to supply the EGR gas. Therefore, it is feared that the fuel efficiency may be deteriorated and/or the discharge amount of NOx may be increased.
Further, if the structure of the intercooler is modified into such a structure that the condensed water is easily discharged, then it is feared that any limitation may arise when the intercooler is carried on a vehicle, and/or the cost may be raised due to the increase in the number of constitutive parts.